The present invention relates to packaged respiring foodstuffs and improvements in the art of packaging foodstuffs which produce gas, particularly CO.sub.2 respiring foodstuffs, especially cheeses such as for example emmental, gouda and edam.
Many hundreds of different kinds of cheese are made today. The cheese making art is very old with evidence of cheese making as far back as 2300 B.C. Cheese is a cultured milk product i.e. typically a starter culture of bacteria which produce lactic acid as added to milk along with an enzyme called "rennin". Rennin typically comes from rennet from the stomach of a calf or lamb, but may be derived from either animal or plant sources. The acid produced by the bacteria alters the pH of the milk to an acidity which causes a milk protein termed "casein" to coagulate thereby forming curds. Rennin is an enzyme which facilitates curd formation. Typically, both acid produced by bacteria and rennin are used together to form cheese curds and whey. Curds aggregate holding fat and whey in a network of protein. In cheese making this curd formation is usually followed by pouring off the whey and concentration of the curds. To remove additional whey, curds may be cut, pressed, cooked and/or salted to produce what is termed "green" or unripened cheese. Here "green" refers to the youth or lack of aging of the cheese at this point in manufacture. The green cheese may then be aged or ripened for anywhere from a few days to up to four years or more depending upon the cheese variety. This ripening may continue even after packaging, but is generally slowed by holding cheese at lower refrigeration temperatures.
The above description relates to generally known processes for making natural cheeses. Also known are "processed" cheeses which are ground natural cheeses which typically mix unripened and ripened cheeses with other ingredients such as added milk and stabilizers followed by pasteurization and usually packaging while hot.
In forming natural cheeses, specific molds or bacteria may be added just prior to or during ripening to produce particular varieties of cheese having different characteristics such as flavors, aromas, textures and appearance.
For example, blue cheeses are made by inserting a blue green mold, Penicillium roquefort into the interior of the cheese. There are also surface ripened cheeses such as brief and camembert which have an exterior surface coat of a white mold Penicillium camembert. Cheeses such as brick and limburger are ripened by bacteria which are coated on the surface of the cheese. The original starter culture bacteria also may provide distinctive characteristics for ripening. Bacteria added in the starter culture is used for ripening in production of hard and semi-hard cheeses such as parmesan, cheddar and gouda. Swiss type cheeses may also be ripened using the original starter culture, but typically additional bacteria such as Propionibacter shermanii is added to form the "eyes" of the cheese. In emmental or Swiss-type cheeses these "eyes" are formed as gas pockets of carbon dioxide (CO.sub.2) which is given off in large amounts by the bacteria which is nourished by lactic acid (which is produced by other bacteria in the starter culture). On grading of Swiss-type cheese, cheese graders (which may be licensed by various governmental entities) consider the amount, size and development of eyes as well as the cheese appearance including uniformity of firmness, and its flavor and aroma, shape, freedom from mold, color, size and saltiness.
After ripening, or after molding and pressing (for starter culture ripened varieties of cheeses), cheeses are coated or packaged to prevent physical damage, moisture loss and spoilage (eg. by mite infestation or growth of undesirable molds or bacteria). Many packaging materials and preventive coatings are in use for contact with cheeses including: fat, cloth, wax, metal foils and plastic films and sheets. Waxes and resins in particular have been used for many years to coat dry, hard or semi-hard cheeses such as cheddar, cheshire, gouda, edam and danbo by dipping the cheese into melted wax. Cheese has also been packaged into polymer film under conditions which allow ripening of the cheese in the package.
In discussing plastic film packaging, various polymer acronyms are used herein and they are listed below. Also, in referring to blends of polymers a colon (:) will be used to indicate that the components to the left and right of the colon are blended. In referring to film structure, a slash "/" will be used to indicate that components to the left and right of the slash are in different layers and the relative position of components in layers may be so indicated by use of the slash to indicate film layer boundaries. Acronyms commonly employed herein include:
PE--Polyethylene (an ethylene homopolymer and/or copolymer of a major portion of ethylene with one or more .alpha.-olefins) PA1 EVA--Copolymer of ethylene with vinyl acetate PA1 PVDC--Polyvinylidene chloride (also includes copolymers of vinylidene chloride, especially with vinyl chloride) PA1 EVOH--A saponified or hydrolyzed copolymer of ethylene and vinyl acetate PA1 EAA--Copolymer of ethylene with acrylic acid PA1 (1) substantially moisture proof i.e. having relatively low moisture vapor transmission rate to prevent drying out PA1 (2) slightly permeable to carbon dioxide to permit normal curing PA1 (3) cling or stick to cheese to prevent mold growth PA1 (4) slightly extensible to improve cling between wrapper & cheese by overfilling PA1 (5) transparent or translucent to improve appearance. PA1 Tensile Strength: ASTM D-882, Method A PA1 % Elongation: ASTM D-882. Method A PA1 Haze: ASTM D-1003-52 PA1 Gloss: ASTM D-2457, 45.degree. angle PA1 1% Secant Modulus: ASTM D-882, Method A PA1 Oxygen Gas Transmission Rate (O.sub.2 GTR): ASTM D-3985-81 PA1 Water Vapor Transmission Rate (WVTR): ASTM F 1249-90 PA1 Elmendorf Tear Strength: ASTM D-1992 PA1 Gauge: ASTM D-2103 PA1 Melt Index: ASTM D-1238, Condition E (190.degree.) PA1 Melting point: ASTM D-3418, DSC with 5.degree. C./min heating rate PA1 0.5 seconds impulse time (upper ribbon only) PA1 2.2 seconds cooling time PA1 50 psi (345 kPa) jaw pressure PA1 0.3 gallon per minute (1 liter per minute) of cooling (about 75.degree. F. (22.degree. C.)) water flow
Various published patent documents disclose different types of cheese packages, packaging films and processes for packaging.
U.S. Pat. No. 1,925,443 (Gere) discloses flexible wrappers and a process for packaging uncured cheese wherein the cheese ripens or cures in the package. This patent states that "The package must be of moisture-proof and impervious material, and it must be so sealed as to exclude air, but at the same time, it must provide for the escape of excess carbon dioxide evolved in the course of fermentation". Preferred wrappers include "cellulose viscose" or "cellulose acetate" which may subsequently be coated with paraffin. Disadvantageously, manufacture of these films is complex, time consuming and expensive. Also, it is difficult to adjust CO.sub.2 permeabilities for use on different cheeses.
U.S. Pat. No. 2,494,636 (Stine) discloses a method of making emmental (Swiss) cheese which comprises applying a coat of extensible, flexible, fluid proof sealing material to the exterior surface of the uncured cheese to seal the surface prior to eye development followed by curing under controlled pressure in an expandable mold. Suitable sealing materials are said to be wax, or a wrap of an elastic-flexible material such as cellophane, the inner surface of which may be coated with a flexible and elastic wax. The packaging materials disclosed here have the same disadvantages as described above for those materials disclosed in the Gere patent.
U.S. Pat. No. 2,871,126 (Smith et al.) discloses a method for manufacturing emmental type cheese which is also known as Swiss cheese. This patent refers to use of thermoplastic film as a moisture proof, fluid-proof material for wrapping the cheese after the brine step for curing in molds. A disadvantage of this disclosed film is that the moisture proof wrapper does not have an adjustable CO.sub.2 permeability.
U.S. Pat. No. 2,813,028 (Jackson, Jr.) discloses processes for producing cheddar cheese. In one process green cheddar curd is extruded into preformed wrappers which may be made of cellulose based films such as cellophane, rubber chloride based films or polyvinylidene chloride based films such as saran. It is preferred that the films have the following characteristics:
The disclosed films suffer from disadvantageously, controlling CO.sub.2 permeability by slightly opening the ends of the package. This removes the physical, moisture and oxygen barrier at those openings thereby subjecting the cheese to the deleterious effects of excessive oxygen, loss of moisture and exposure to the environment.
Canadian Patent Application 2,053,707 (Mueller) discloses laminate films for packaging soft cheeses such as camembert and brie. Known materials for packaging such soft cheese is said to include polyethylenes with and without ethylene vinyl-acetate copolymers, polypropylenes, nylon/polyethylene laminates, and polyester/polyethylene laminates. Oxygen and carbon dioxide transmission rates are said to be "of primary importance in the packaging of many soft cheeses, as well as other foods items which require a packaging material of high gas permeability such as many fruits and vegetables". (See page 1). The disclosed film of Mueller comprises a first film component (which is perforated) laminated to a gas permeable layer which include at least one layer comprising butadiene styrene copolymers. Relative gas and moisture transmission rates are said to be determined by the size and number of perforations in the first layer as well as the thickness and permeability of the second layer.
In the examples, permeabilities of the film of Example 3 are stated as follows:
"The water vapor transmission rate averaged about 2.73 g/100 in.sup.2, 24 hr. at 100.degree. F. and 100% RH. The oxygen transmission rate averaged about 4858.9 cm.sup.3 /m.sup.2 atm., 24 hrs. at 73.degree. F. The carbon dioxide transmission rate averaged about 30204.0 cm.sup.3 /m.sup.2, atm., 24 hrs. at 73.degree. F."
These films have a very high permeability to oxygen as well as carbon dioxide and such extremely high oxygen permeability while perhaps suitable for mold cured cheeses is undesirable for hard or semi-hard cheeses such as emmental, gouda, edam and the like due to the possibility of facilitating undesirable mold growth.
Canadian Patent Application No. 2,050,837 (Gillio-Tos et al.) discloses polymer mixtures of polyvinylidene chloride and polyethyloxazoline which are purportedly useful in forming monolayer or multilayer films having increased moisture permeability with no substantial change in permeability to oxygen or carbon dioxide. This combination of properties purportedly is "indicative of utility in packaging, for example, medical applications, casings and the curing of non-gassing cheeses such as parmesan" (page 3, last paragraph). A table shows moisture, oxygen and carbon dioxide permeability rates. These films are made from chlorinated polymers which are increasingly more difficult to dispose of or recycle as further discussed below.
EP 457 598 (Shah et al) discloses a polyamide based multilayer film for packaging cheese. This polyamide film is disclosed as having "an oxygen transmission rate of no more than 500 cc/m.sup.2, 24 hrs., atm and a carbon dioxide transmission rate of at least 750 cc/m.sup.2, 24 hrs., atm.". Example 5 purportedly discloses a 1 mil (25.4 micron) thick biaxially oriented film having a core layer comprising a blend of about 70% EVOH and about 30% of a polyamide in combination with polypropylene or propylene copolymer based outer layers and this film has a reported shrinkage at 220.degree. F. (104.degree. C.) of 24% in two directions. The core layer is about 14% of the thickness of the film which would be 0.14 mil (3.6 microns). Example 8 purportedly had outer layers of 90% linear medium density polyethylene blended with 10% of an EVA-based masterbatch and a core layer which was a blend of 70% nylon and 30% EVOH, with the core layer comprising 25% of the total film thickness. Disadvantageously, the shrinkage values of this film are achieved at high temperatures with the lowest reported measurement being made at 104.degree. C. as denoted in the Table on page 8, and it is therefore to be expected that the shrinkage values at 90.degree. C. and lower temperatures would disadvantageously be much less. This results in films having high shrinkage only at undesirably high temperatures.
Various monolayer and multilayer thermoplastic films have been commercialized for packaging cheeses. Three to five layer films are common. Typical structures include: EVA/PVDC/EVA, EVA/EVA/PVDC/EVA, Ionomer/EVA/PVDC/EVA, and variations thereof where ethylene based polymers are blended into one or more of the EVA layers. Some cheese packaging films are heat shrinkable at 90.degree. C. and others are not. Some of the nonshrinking films have an oxygen barrier comprising one or more layers of nylon or EVOH or a blend of EVOH with nylon. Such known nonshrinking films include structures of the type EVA:PE/Nylon, EVA:PE/Nylon/EVOH/Nylon/EVA:PE, EVA:PE/PVDC/Nylon, EVA:PE/EVOH/Nylon, and EVA:PE/Nylon/EVA. The known nonshrinking EVOH containing films generally have a relatively thick EVOH containing layer, generally greater than 0.5 mil (12.7 microns).
Of the foregoing nonshrinking films, those containing EVOH have a typical oxygen permeability of less than 10 cm.sup.3 per m.sup.2 at 1 atm, 0% relative humidity and 23.degree. C. and are considered high barrier films. The terms "barrier" or "barrier layer" as used herein mean a layer of a multilayer film which acts as a physical barrier to gaseous oxygen molecules. Physically, a barrier layer material will reduce the oxygen permeability of a film (used to form the bag) to less than 70 cm.sup.3 per square meter in 24 hours at one atmosphere, 73.degree. F. (23.degree. C.) and 0% relative humidity. These values should be measured in accordance with ASTM standard D-1434.
Also known are films suitable for packaging cheese that are heat shrinkable at 90.degree. C. which contain nylon or a blend of EVOH and nylon. Axially stretched, especially biaxially stretched, films which are "heat shrinkable" as that term is used herein have at least 10% unrestrained shrinkage at 90.degree. C. (10% in both the machine direction (M.D.) and transverse direction (T.D.) for biaxially stretched films). Such known films include structures of the following types: Ionomer/PE/Nylon, Ionomer/EVA/Nylon, EAA/Nylon:EVOH/Ionomer, and PE/EVOH:Nylon/PE. Some of these EVOH containing heat shrinkable films have an oxygen permeability in the high barrier range. A few heat shrinkable, EVOH-containing films have permeabilities which are outside the high barrier range such as e.g. about 30-35 cm.sup.3 /m.sup.2 or even as high as 150-170 cm.sup.3 /m.sup.2 at 1 atm, 0% relative humidity and 23.degree. C.
As shown in the present specification, high barrier film (whether shrinkable or not) which are very good oxygen barriers typically also have very low carbon dioxide permeabilities which may be disadvantageously low for packaging respiring articles such as cheeses, particularly hard and semi-hard cheeses. Packaging films which have low permeability to CO.sub.2 are subject to pillowing when hermetically sealed around an enclosed respiring article. If the respiration rate of the enclosed article exceeds the CO.sub.2 transmission rate for permeating the enclosing film, "pillowing" will occur. Pillowing or "ballooning" refers to the inflation of the sealed film which typically causes the film surface to move away and out of contact with much of the surface of the enclosed article. For such respiring articles as foodstuffs e.g. hard and semi-hard cheeses, it is perceived that some customers view pillowing as a defect and avoid purchase of refrigerated foodstuffs having a pillowed container. Furthermore, it is believed that retention of high concentrations of CO.sub.2 about a respiring foodstuff may possibly adversely affect the curing process itself, possibly delaying development of the desirable characteristics of the microbiological processes including e.g. full flavor and aroma development.
Also, the prior art EVOH-containing high permeability cheese films have several disadvantages for packaging respiring cheeses including one or more of the following: undesirably low shrink values particularly at low temperatures e.g. 90.degree. C. or lower, an undesirably narrow heat sealing range, use of expensive resins such as ionomer in the other layers, and poor optical properties such as high haze, low gloss and/or streaks or lines which detract from the film appearance. Furthermore, known EVOH-containing cheese films have a disadvantageously thick EVOH-containing layer which is often 2 to 10 times thicker than the present invention and difficult to process into an oriented film, difficult to make into a heat shrinkable film having high shrinkage values and shrink forces, especially in two directions, and which requires more material which may increase expense.
As shown by the above, many different multilayer film structures have been and continue to be commercially made and used to package cheeses. These structures all suffer from various disadvantages, especially with respect to packaging "respiring" cheeses i.e. those cheeses which give off CO.sub.2 gas.
For example, higher levels of CO.sub.2 permeabilities with PVDC containing films require that the PVDC layer be heavily plasticized to achieve gas permeability. Such plasticizers may adversely affect other film properties including processability, optical properties, and orientability.
Also, recycling of PVDC polymers is difficult, particularly where the waste polymer is mixed with other polymers having different melting points. Attempts to remelt film containing PVDC frequently results in degradation of the PVDC component.